When a conductor loop or coil carrying radio frequency current is wrapped around a dielectric chamber containing gas at low pressure, a plasma is formed inside the chamber as a result of currents induced by the changing magnetic field of the coil known as inductive coupling. Plasmas may be produced inside metal-walled chambers, if the chambers are suitable for high power CVD diamond reactors, which is the subject of a recent patent application U.S. Ser. No. 08/483,982, to A. E. Robson, et al, for a Durable Plasma Treatment apparatus and Method.
Although the primary advantage of metal chambers is to allow much grater power to be dissipated in the plasma than is possible in dielectric chambers, there is a secondary advantage to slotted metal chambers: the chamber acts as a Faraday cage and prevents capacitive coupling between the coil and the plasma. As is well known, capacitive coupling can lead to significant potentials arising between the coil and the plasma. As is well known, capacitive coupling can lead to significant potentials arising between the plasma and the wall of a dielectric chamber, and the resulting acceleration of plasma ions toward the walls can be deleterious to many processes.
In the case of metal walled chambers, the maximum potential between the plasma and the wall is limited to the "floating potential," V.sub.f, given by ##EQU1## where T.sub.e, is the electron temperature, m.sub.i is the ion mass and m.sub.e is the electron mass. For a hydrogen plasma at a typical temperature of 2eV, V.sub.f .apprxeq.5.7V; in a water plasma, where the principal ion is H.sub.3 O.sup.+, V.sub.f .apprxeq.8.6 V. Ions accelerated to the walls by these potentials will have little or no deleterious effect on the deposition process. On the other hand, these potentials are sufficient to sustain unipolar arcs on the chamber walls and these can be highly deleterious to any process because they introduce significant quantities of wall material into the plasma. There appears to be evidence that unipolar arcs are occurring in the 3M2 reactor.
Unipolar arcs can occur in metal chambers whenever V.sub.f &gt;V.sub.c, where V.sub.c is the cathode fall of an arc. V.sub.c depends on the cathode material and is in the range 7-12 V. Further details may be had by reference to the paper by A. D. Robson and P. C. Thonemeann, entitled "An Arc Maintained on an Isolated Metal Plate Exposed to a Plasma," Proc. Phys. Soc. (London) 73, 508, (1959), which is incorporated herein by reference. Ways of eliminating unipolar arcs are discussed in the paper by A. E. Robson and R. Hancox, "Choice of Materials and Problems of Design of Heavy Current Toroidal Discharge Tubes," Proc. IEE (London) 106A, Suppl. 2, 47 (1959). It seems unlikely that the methods described in this paper, which is concerned with pulsed fusion systems, could be applied to the unipolar arcs in 3M2. On the other hand, a method that is not available in fusion systems is applicable to inductively coupled r.f. plasmas, namely: the direct control of the plasma floating potential V.sub.f, which is the subject of the present invention.